1. Field of the Invention
The present invention pertains to providing two dimensional models of the subsurface physical properties of the earth and more particularly to a method for translating a drawn or imaged model into a numerical format for use in computer calculations.
2. Related Prior Art
Numerical models of rock properties such as velocity are widely used by geophysicists to generate synthetic seismic data and perform seismic image reconstruction.
There are practical problems concerning how a layered earth model can be specified to ensure that no ambiguity exists. Since the model is created for the purpose of numerical calculation, this also introduces practical constraints on how the model must be constructed. The traditional solution has been to set strict rules governing the interface data used to construct the model and the order in which it is input.
There are several commercially available seismic modeling software packages, all of which include a model building element. One choice that is typical in its approach to model building is representing the model as a series of layered interfaces. It achieves the same end as the present invention However, to ensure that there is no ambiguity and for numerical expediency, this system requires that layers be entered in strict sequence, that each layer extends across the entire model and that fictitious layers are needed to create overhanging structures. To comply, the user must enter artificial interface data in addition to the actual horizon data and must take car in organizing it.
Many oil companies, geophysical service companies, universities, and research institutions have their own model building processes. The "layer" approach is common but "cell" or "polygon" models are also used.
An example of a commonly used modeling technique is illustrated in U.S. Pat. No. 4,679,174, "Method for Seismic Lithologic Modeling", issued to Valery A. Gelfand, which describes a method of seismic exploration of the subsurface of the earth. Seismic reflection data are gathered in a selected area. The seismic data are combined with available non-seismic data to define an initial two dimensional lithologic model. Based upon the initial model, a set of synthetic seismic data is generated. The degree of correspondence between the set o synthetic reflection data and the gathered seismic data is determined. The initial model-parameters are systematically perturbed during a series of interactions until a desired degree of correspondence has been achieved, resulting in a final lithologic model.
Other United States patents which relate to seismic data processing techniques that relate generally to modeling are as follows.
U.S. Pat. No. 4,415,999, "Method of Confirming Seismic Data Interpretation", issued to George P. Moeckel et al., describes a method of generating synthetic seismograms for use in determining the accuracy of hypothesized subterranean structures. The method features defining hypothesized detector location corresponding to real detector locations rather than interpolating from arbitrary detector locations generated by specifying of initial ray path angles as input data. The accuracy of the results is obtained with the elimination of interpolation.
U.S. Pat. No. 4,747,054, "Method for Non-Linear Signal Matching", issued to Chittibabu Chittineni, describes a process for non-linear matching of time analog signals, particularly seismic signals, that have generally similar response characteristics but different time bases. The process models the signals to be matched as a stretched version of each signal with additive noise, and a match curve is estimated adhering to a global criterion of maximization of the likelihood function. The global criterion is further optimized in accordance with the principle of path optimality and other important information may also be considered. The match curve may then be used to identify such as correlated signal events, comparison of signal sections, and numerous seismic data processing functions using multi-sensor time analog data input.
U.S. Pat. No. 4,766,574, "Method for Depth Imaging Multicomponent Seismic Data", issued to Norman D. Whitmore, Jr. et al., describes a method for imaging multicomponent seismic data to obtain depth images of the earth's subsurface geological structure as well as estimates of compressional and shear wave interval velocities. In particular, measures are obtained of imparted seismic wave fields incident on reflecting interfaces in the earth's subsurface and of resulting seismic wavefields scattered therefrom. The incident and scattered seismic wavefields are employed to produce time dependent reflectivity functions representative of the reflecting interfaces. By migrating the time dependent reflectivity functions, depth images of the reflecting interfaces can be obtained. For a dyadic set of multicomponent seismic data, the dyadic set of multicomponent seismic data are partitioned so as to separate the variously coupled incident and reflected wavefields in the recorded multicomponent seismic data. The incident and reflected wavefields are cross-correlated to form time dependent reflectivity functions. The time dependent reflectivity functions are then iteratively migrated according to a model of wavefield velocities of propagation to obtain estimates of the compressional and shear wave interval velocity. The migrated reflectivity functions can then be stacked to produce depth images of the earth's subsurface geological structures.
U.S. Pat. No. 4,821,242, "Depositional Reconstruction for Petroleum Location", issued to Willard M. Hennington, describes an exploration development technique for delineating hydrocarbon accumulations and prospect development. The technique uses existing information and establishes a relationship between present day subsurface structure and stratigraphic prehistoric reservoir development for determination and projection of relative reservoir development, definition and grading of combination structural-stratigraphic type traps, determination of permeability barriers, migration paths, accumulation areas, prospect grading, pressure cells and prediction of fluid movement. Depositional reconstruction results in a summary map which outlines productive areas, shows potential extensions, graded prospects, relative reservoir developments, and the other stratigraphic conditions which control the successful economic exploitation of a potential horizon.
However, all of the foregoing patents illustrate modeling methods that require the use of seismic data or processing techniques in order for their methods to be operative.